High voltage power supplies often are used in applications where a load connected to the power supply includes a significant inductive or capacitive component. In many applications, inductive components comprise a major component of the load. For example, electrostatic precipitators contain electrode arrays connected to a high voltage power supply via high voltage cables and buses. The array, the cables, and the buses all contribute to the overall inductive value of the system. Even in applications that do not contain large inductive components, elements of the system may have non-negligible inductive values. For example, a load connected to a high voltage power supply may be connected to the power supply by a high voltage cable having an inherent inductance. As a general rule, therefore, all high-power electrical circuits can be modeled with resistive, inductive, and capacitive components.
The presence of capacitive and inductive load components often makes it difficult to maintain a desired current in the load because the components prevent the current in a load from quickly changing. This is a disadvantage because in many applications, it is beneficial to maintain the current in a high power circuit at a desired level. A constant current allows the transfer of a maximum amount of electrical energy in a minimum time. A limited amplitude also increases the safety of those exposed to the circuit. Finally, a limited amplitude is ideal for power supplies constructed of power transistors. Since power transistors are rated to handle a maximum current, transistors used in power supplies that generate current waveforms that may periodically exceed an average current level must be selected to handle the periodic spikes to a much higher current level. If the amplitude of a load current is controlled to remain around an average level, typically less expensive power transistors having a lower current carrying capability may be selected for use in the power supply.
Several different approaches exist for designing a power supply that produces a direct current (DC) high voltage. For example, a common approach is to convert a DC voltage to a high frequency alternating current (AC) square wave voltage with an inverter, step-up the square wave voltage using a high voltage transformer, and rectify the stepped-up output of the transformer to produce an approximately DC output voltage. A drawback to this generation method is that it does not allow simple control of the magnitude and waveform of the current generated in the transformer and in the load. The magnitude of the current in the load is affected by two changing voltages. First, fluctuations in the primary DC voltage source can cause the amplitude of the AC square wave voltage from the inverter to vary. Variance in the output from the inverter will have a direct effect on the output voltage from the power supply. Second, changes in the voltage drop across the inductive or capacitive components in the load will affect the current through the load if the output voltage from the power supply remains constant. For example, in precipitator applications, the voltage drop across the electrode array in the precipitator will fluctuate in response to varying amounts of pollutants passing through the precipitator. The two changing voltages, i.e., the magnitude of the output voltage generated by the high voltage power supply (hereinafter V.sub.gen) and the voltage across the capacitive components in the load (hereinafter V.sub.c), often change independently of each other. The variation of both V.sub.gen and V.sub.c effects the rate of change of current in the load. Because prior art power supplies were not successful in rapidly controlling the difference between the output voltage from the power supply and the voltage in the load, the magnitude and waveform of the current in the supply and load often went uncontrolled.
Generating a controlled current in an electrical circuit that includes inductive and capacitive components is therefore a difficult problem. Several solutions have been suggested, all of which limit the switching; speed of the power supply. For example, one solution is to convert the unstable DC input voltage to a stable current by adding a current regulator or limiter to the output of the power supply. While an added regulator will limit the output current from the high voltage power supply, a current regulator also contains reactive elements which store energy. When the power supply is turned off, the stored energy must dissipate through other components, preventing the output of the supply from rapidly decreasing to zero. Those techniques which allow the production of a regulated current from a high voltage power supply therefore have a tendency to prevent the rapid shut down of the power supply. It is an object of the present invention to create a power supply that produces a regulated current in a load and allows rapid power down.